The invention relates to load cells, and in particular to load cells for use in sensing weight and position of a seated occupant in a motor vehicle for deployment of safety devices, such as air bags.
Since the development of the air bag and its inclusion in automobiles a problem has existed with the relative deployment force used for various individuals. Air bags have been a requirement on new vehicles since 1992. Air bags are made to arrest the forward momentum of the driver or passenger in the event of a collision. If one designs a universal air bag for all passengers, then it must have sufficient force to stop the largest of the expected passengers. Smaller passengers have less momentum, and so do not require the same momentum change as the larger passenger. In addition, smaller passengers are shorter, and sit closer to the dashboard, and therefore experience more of the bag""s explosive force than a average adult male, sitting further back. As a result of the current air bag deployment force, there have been a number of injuries and fatalities associated with air bag deployment. As of mid 1998, 105 deaths have been attributed to the deployment of air bags with a small adult or a young child when no air bag deployment would not have resulted in any injury to the occupant.
This situation has caused NHTSA, the National Highway Traffic Safety Administration, a branch of the U.S. Department of Transportation, to propose rules which will change the criteria for air bag activation, as well as the deployment force, in order to protect such small occupants. In addition to these requirements, the NHTSA has also identified xe2x80x9cout of positionxe2x80x9d occupants as a source of concern. Thus a system must be able to modulate or reduce the air bag deployment force if the occupant is in a position so as to be injured by the air bag, even if that occupant is a full size adult.
There are several methods which can sense the presence and weight of an occupant. In U.S. Pat. No. 5,573,269, Gentry et al. teach an apparatus which uses weight measurements, using a sensor, in an automobile seat as an input to a controller which operates air bags. This sensor, described in U.S. Pat. No. 5,494,311, is a thin structure that resides in the bottom seat cushion. As is recognized by Gentry, much of the occupant""s weight is also directed into the seat back, thereby bypassing the weight sensing pad and traveling directly through the seat structure to the chassis of the vehicle. An incline sensor, which measures the tilt of the back of the seat is also provided to compensate for this effect.
There are two problems with this system. First it assumes that the weight can be determined only by the pressure on the seat cushion bottom and by the angle of the seat. That is not always the case. Consider an occupant who puts horizontal pressure on the floorboard in front of the seat. This increases the force on the back with a resulting decrease on the bottom cushion. At some point this pressure can be great enough that nearly all of the occupants weight is on the back cushion. This problem is also present in U.S. Pat. No. 5,474,327. In this device a set of pressure sensitive pads is placed beneath the surface of the seat cushion. While this device is adequate for the detection of a child seat, it does not give adequate information for small adults and out of position occupants.
Blackburn et al. teaches in U.S. Pat. No. 5,494,311 a system where pads are placed in both the lower and rear seat cushion. This gives a better weight measurement under all conditions, the obvious downside is the cost.
One of the problems of prior systems is that they cannot read negative weight, i.e. when forces are present that would cause the force on the seat support to go negative. This can occur when the occupant places force, via his feet, on the front of the passenger compartment.
Yet another difficulty is that since the pressure is sensed on the seat, the seat belt tension adds to the reading. A 40 pound car seat could then, with sufficient tension on the seat belt, put 200 pounds of force on the seat surface, causing a false reading.
An object of the invention is to devise an apparatus for accurately sensing weight of an occupant in an automotive seat for deployment of restraint devices.
Another object of the invention is to determine where a passenger is seated in an automotive seat.
The above object has been achieved with a torsional sensing load cell having the shape of a tuning fork with two arms. The load cell is configured to handle overload by the use of a deflection stop pin. For example, in an automotive application, one arm of the cell supports part of the load of a car seat and the other arm is fixed to a foot attached to the automotive chassis. Torsion exists in the load cell as the load arm deflects relative to the fixed arm. A pair of strain gauges measure the torsion in the load cell and produce an electrical signal which is reported to a circuit which converts the electrical signal to a weight measurement. By placing a load cell at each of four corners where car seat support feet are located, the entire load in a car seat can be measured and the position of a seated person can be determined by observing weight distribution among the four corners of the seat. Since the support feet are insensitive to the manner in which loads are generated, the load cells sense true load, even where unexpected loads are created, for example by a car passenger pushing against a dashboard by means of his feet.
An automotive car seat is usually moveable by means of an electric motor and is not directly mounted to the automobile chassis. Rather, the car seat is mounted on two parallel moveable glide rails which are movably supported on rollers by two parallel fixed guide rails. The guide rails are fixed in place by rigidly connecting each guide rail between two support feet, one at the front of a seat and one at the back. The moveable glide rails transmit force to the fixed guide rails. Since the load cells of the present invention link the fixed guide rails to the fixed feet, torsion is allowed to develop between a guide rail and a fixed foot. Torsion then exists in the bridge section of the load cell, between the two arms of the load cell. Here is where strain gauges are mounted for torsion measurement. Electrical signals generated by the strain gauges ar sent to a circuit which produces a force signal. Signals from four load cells associated with an automotive seat are directly proportional to the weight of an occupant in the seat. The fractional distribution of weight between forward load cells associated with the front of the seat and rearward load cells associated with the rear of the seat indicate where an occupant is seated.